Random access procedure for handover

1. A multi-radio access technology (multi-RAT) user equipment (UE) operable to switch from a wireless local area network (WLAN) to a wireless wide area network (WWAN), the UE having one or more processors configured to:

receive WLAN-specific dedicated physical random access channel (PRACH) allocation information from an evolved node B (eNB) to enable the multi-RAT UE to perform an inter-RAT WLAN-to-WWAN handover;

initiate the inter-RAT WLAN-to-WWAN handover at the multi-RAT UE by performing random access with the eNB using the WLAN-specific dedicated PRACH allocation information; and

wherein the WLAN-specific dedicated PRACH allocation information includes a time division multiplexing (TDM)-based dedicated PRACH allocation to enable a plurality of multi-RAT UEs to each initiate an inter-RAT WLAN-to-WWAN handover using a same PRACH preamble, a subframe index, and a frame index per cycle, wherein the PRACH preamble is a preamble shared among a group of UEs.

2. The multi-RAT UE of claim 1 , wherein the one or more processors are further configured to perform non-contention based random access or contention-based random access with the eNB using the WLAN-specific dedicated PRACH allocation information.

3. The multi-RAT UE of claim 1 , wherein the WWAN network includes a Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8, 9, 10, 11 or 12 network.

4. The multi-RAT UE of claim 1 , wherein the one or more processors are further configured to initiate the inter-RAT WLAN-to-WWAN handover by performing at least one of time division multiplexing (TDM) or code division multiplexing (CDM) random access using the WLAN-specific dedicated PRACH allocation information.

5. The multi-RAT UE of claim 1 , wherein the one or more processors are further configured to receive WLAN-specific PRACH configuration information that is broadcast from the eNB via a system information block type 2 (SIB2) message, the WLAN-specific PRACH configuration information to be used for performing the inter-RAT WLAN-to-WWAN handover for the multi-RAT UE using the same PRACH preamble.

6. The multi-RAT UE of claim 5 , wherein the WLAN-specific PRACH configuration information includes a root sequence number for the WLAN-to-LTE handover, a PRACH configuration index for the WLAN-to-LTE handover, a high speed flag for the WLAN-to-LTE handover, a zero correlation zone configuration for the WLAN-to-LTE handover, and a PRACH frequency offset for the WLAN-to-LTE handover.

7. The multi-RAT UE of claim 1 , wherein the one or more processors are further configured to receive the WLAN-specific dedicated PRACH allocation information from the eNB in response to sending a handover request message to the eNB, the handover request message for switching the multi-RAT UE using the same PRACH preamble from operating over the WWAN to operating over the WLAN.

8. The multi-RAT UE of claim 1 , wherein the one or more processors are further configured to receive the WLAN-specific dedicated PRACH allocation information in a handover response message, the multi-RAT UE being configured to switch to operating over the WLAN upon receiving the handover response message from the eNB.

9. The multi-RAT UE of claim 1 , wherein the one or more processors are further configured to initiate the WLAN-to-WWAN handover at the multi-RAT UE by performing at least one of time division multiplexing (TDM) or code division multiplexing (CDM) random access using a PRACH preamble included in the WLAN-specific dedicated PRACH allocation information, a subframe index included in the WLAN-specific dedicated PRACH allocation information, and a frame index per cycle included in the WLAN-specific dedicated PRACH allocation information.

10. The multi-RAT UE of claim 9 , wherein the PRACH preamble is a unique preamble for the UE.

11. The multi-RAT UE of claim 1 , wherein the WLAN-to-WWAN handover at the UE occurs in an integrated WLAN and WWAN network.

12. A method for facilitating wireless local area network (WLAN) to wireless wide area network (WWAN) handover, the method comprising:

sending WLAN-specific physical random access channel (PRACH) configuration information from an evolved node B (eNB) to a user equipment (UE), the WLAN-specific PRACH configuration information for WLAN-to-WWAN handover at the UE;

receiving, at the eNB from the UE, a handover request message to switch the UE from operating over the WWAN to operating over the WLAN;

sending a handover response message from the eNB to the UE, the handover response including WLAN-specific dedicated PRACH allocation information, the UE transitioning into operating over the WLAN upon receiving the handover response message from the eNB; and

facilitating the WLAN-to-WWAN handover for the UE, the UE initiating the WLAN-to-WWAN handover by performing at least one of time division multiplexing (TDM) or code division multiplexing (CDM) based random access using the WLAN-specific dedicated PRACH allocation information wherein the WLAN-specific dedicated PRACH allocation information includes the time division multiplexing (TDM)-based dedicated PRACH allocation to enable a plurality of multi-RAT UEs to each initiate an inter-RAT WLAN-to-WWAN handover using a same PRACH preamble, a subframe index, and a frame index per cycle, wherein the PRACH preamble is a preamble shared among a group of UEs.

13. The method of claim 12 , further comprising sending the WLAN-specific PRACH configuration information via a system information block 2 (SIB2) message that is broadcast to the UE.

14. The method of claim 12 , wherein the WLAN-to-WWAN handover is triggered at the UE in response to WLAN interference that exceeds a defined threshold.

15. The method of claim 12 , wherein the WLAN-specific PRACH configuration information includes a root sequence number for the WLAN-to-LTE handover, a PRACH configuration index for the WLAN-to-LTE handover, a high speed flag for the WLAN-to-LTE handover, a zero correlation zone configuration for the WLAN-to-LTE handover, and a PRACH frequency offset for the WLAN-to-LTE handover.

16. The method of claim 12 , wherein the handover request message received from the UE to switch from WWAN to WLAN includes a number of data resource blocks (DRBs) offloaded to a WLAN link and associated DRB identifiers (IDs).

17. The method of claim 12 , further comprising receiving an additional handover request message from the UE to switch from WLAN to WWAN, the additional handover request message including a number of data resource blocks (DRBs) offloaded to a WWAN link and associated DRB identifiers (IDs).

18. A user equipment (UE) operable to switch from a wireless local area network (WLAN) to a wireless wide area network (WWAN), the UE comprising:

a communication module configured to receive time division multiplexing (TDM)-based WLAN-specific dedicated physical random access channel (PRACH) allocation information from an evolved node B (eNB) to enable the UE to perform the WLAN-to-WWAN handover, wherein the communication module is stored in a digital memory device or is implemented in a hardware circuit; and

a handover module configured to initiate the WLAN-to-WWAN handover at the UE by performing TDM-based non-contention based random access with the eNB using the WLAN-specific dedicated PRACH allocation information, wherein the WLAN-specific dedicated PRACH allocation information includes the time division multiplexing (TDM)-based dedicated PRACH allocation to enable a plurality of multi-RAT UEs to each initiate an inter-RAT WLAN-to-WWAN handover using a same PRACH preamble, a subframe index, and a frame index per cycle, wherein the PRACH preamble is a preamble shared among a group of UEs, and the handover module is stored in a digital memory device or is implemented in a hardware circuit.

19. The UE of claim 18 , wherein the WWAN network includes a Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8, 9, 10, 11 or 12 network.

20. The UE of claim 18 , wherein the communication module is further configured to receive WLAN-specific PRACH configuration information that is broadcast from the eNB via a system information block type 2 (SIB2) message, the WLAN-specific PRACH configuration information to be used for performing the WLAN-to-WWAN handover at the UE.

21. The UE of claim 18 , wherein the communication module is further configured to receive the TDM-based WLAN-specific dedicated PRACH allocation information from the eNB in response to sending a handover request message to the eNB, the handover request message for switching the UE from operating over the WWAN to operating over the WLAN.

22. The UE of claim 18 , wherein the handover module is further configured to initiate the WLAN-to-WWAN handover at the UE by performing time division multiplexing (TDM) non-contention based random access using a PRACH preamble included in the WLAN-specific dedicated PRACH allocation information, a subframe index included in the WLAN-specific dedicated PRACH allocation information, and a frame index per cycle included in the WLAN-specific dedicated PRACH allocation information.